How much does stealth fuselage increase cost of Trainer jet? I am not talking about B2's material, i mean, cost of enter level of low RCS material.
Mate, Hürjet is for training pilots before they start flying proper fighters, just as Hürkuş is for people who needs to learn to fly the first time. These planes have to be simple and reliable enough for people to learn to fly, that is their first and foremost purpose. Despite having all the money in the world USAF, for example, still have turboprop trainers (naturally, they are the basic trainers after all) and T-38s and their replacement for T-38 is T-7 which is still an ordinary advanced trainer just like Hürjet.
And, how stealthy a plane is have very little to do with the pilot. It is about its geometrical shape, the materials it was made from and coated with (not just RAM paint but even how well it blends in with background), its sensors, how much heat it gives out, even how loud it is.
Sure, there are differences between flying an F-16 and F-35, but those are because of the difference between two planes. And a pilot who has gone through training and ready to fly an F-35 learns the how to fly that plane, on that plane. A stealthy trainer wouldn't help the pilot.